1. Field of the Invention
This invention relates to a process for manufacturing an improved cookware vessel. The process consists of manufacturing such items from foamable metal bodies.
2. The Prior Art
U.S. Pat. No. 5,151,246 to Baumeister et al. discloses a method for manufacturing foamable metal bodies. This method consists of mixing metal powder and gas-splitting propellent powder and hot compacting the mixture to a semi-finished product at a temperature at which the joining of the metal powder particles takes place primarily by diffusion. The pressure is sufficiently high to hinder the decomposition of the propellant so that the metal particles form a solid bond with one another and constitute a gas tight seal for the gas particles of the propellant. This patent is hereby incorporated by reference.
This invention employs the above described material to produce an improved cookware vessel that can be heated by cooking, broiling, boiling, grilling or baking. The vessel can be manufactured in any size or shape. The method of manufacturing can be accomplished by the use of any metal, or combination of metals. Aluminum, aluminum alloys, copper, all ferrous alloys, cast iron, alloy steel, stainless steel, carbon steel, or iron based super alloys can be used. The vessel may incorporate one metal for the faces and the core or may employ one metal on the faces and another for the core. For example one embodiment may be a stainless steel solid outside shell or fascia bonded to an aluminum metal foam core. The weight of a solid steel shell and solid aluminum core when compared to solid steel shell and porous foam aluminum core can reduce the weight of the cookware by 25-40%. (Finite Element Analysis) FEA indicate that a solid steel outside shell and a core of aluminum foam would result in a similar conductivity to solid steel of the same overall thickness with a significant decrease in weight. Another combination is non-porous aluminum outsider layer or shell with a porous aluminum foam core. This would result in a considerable reduction in weight without compromising conductivity greatly depending on the density of the foamable aluminum and the use for which the vessel is intended.
An advantage of manufacturing a cookware vessel from foamable metals is the significant decrease in the weight of the cookware vessel. FEA analysis was completed with a non-porous metal shell (outside layer) and a core of porous metal foam of various material combinations and thicknesses. Comparative heat conductive and weight analyses have been made on various configurations where metal foam is involved. Optimal thickness of the non-porous shell and metal foam core densities are determined when samples are made for different cooking vessels and tested for optimum conditions. Sandwich structures composed of a porous metallic foam core and metallic face sheets can be produced, with options exploiting combined materials and shapes. Typical foaming processes include casting, powder pressing, powder metallurgy, metallic deposition and sputter deposition. Following the metalworking steps, the foamable material is heated to temperatures near the melting point of the matrix material. During heating, the foaming agent decomposes, and the released gas forces the densified material to expand into a highly porous structure. The density of the metal foams can be controlled by adjusting the content of the foaming agent and several other foaming parameters, such as temperature and heating rate. Different alloys can be foamed by selecting appropriate foaming agents and process parameters. The bonding of a face sheet to the metal foam core would be accomplished by brazing, soldering, diffusion bonding, welding (inert gas welding, laser welding, vibration welding) or roll cladding. Sandwich panels consisting of a foamed metal core and face sheets can be fabricated by bonding the face sheets to the core in the above mentioned methods.
The powder metallurgy production method makes it possible to build metallic foam parts that have complex geometry. Sandwich structures composed of porous metallic foam core and metallic face sheets can also be produced, with options exploiting combined materials and shapes. The material may be foamed into complex shapes by inserting the foamable material into a hollow mold and expanding it through heating. Sandwich panels consisting of a foamed metal core and solid face sheets may be fabricated by bonding the face sheets onto a foam core. If pure metalic bonding is required, face sheets and foamable material can be roll-clad to make a sandwich structure before foaming. The metal foam sandwich process has the advantage of enabling not only flat panels but also true three dimensional shapes by e.g. deep drawing or other forming process prior to the foaming.
The non-porous fascia or surface of the foamable metal product may be hot dipped, electroplated enameled electroless plated, plasma sprayed, vacuum metalized, sputtered, metal powdered, flame sprayed, or treated with any variety of finishes that are acceptable for food services products. The cookware can be used on a gas, convection, electric, or induction heat oven or stove.
A cookware vessel employing a solid, non-porous, outer layer with a foamable metal core is significantly lighter when compared to other metal cookware that combine steel and solid aluminum currently on the market. The FEA results indicate that the porous aluminum core provides better radial heat transfer than the porous steel material, with the solid aluminum providing the best performance. After 100 seconds the temperature differences are more significant with air boundary conditions than in water boundary conditions. Therefore a large pot for boiling water or cooking stews would be suitable employing a porous aluminum core with 50% porosity. When cooking with a limited amount of liquid, the air boundary conditions would apply, requiring a more dense metal foam core.
All FEA analyses within this study used a medium gas flame heat source. Appliances using induction heat, electric resistance and convective heating sources will present relatively constant heat distributions over the entire heating surface of the cookware product, thus resulting in minimal if any xe2x80x9chotspotsxe2x80x9d regardless of the cooking material. FEA analysis simulating induction heat, electric resistance and convective heating were not performed because it was assumed that these results would show a constant heat distribution with minimal hotspots across the cooking surface. The porous material heats up faster than the solid material due to its lower density (thus requiring less energy).
Air boundary conditions present the worst scenario for developing a hot spot. Water boundary conditions, such as soup, water, etc. will not have the severe hot spots as those present in the air environment because these cooking materials will draw the heat out of the cookware material much faster than an air environment only. The air boundary condition may only present during the heating prior to placing the cooking material inside the cookware. Therefore, the water boundary condition results may be of more benefit for evaluating the optimum porous material thickness. Results from the water boundary condition indicate small temperature variations over the heating surface for different porous material thickness. Although if we consider the air boundary conditions, based solely on heat transfer, it can be concluded that the largest amount of porous material should be used on vessels that will hold liquids, soups, etc. to achieve the lightest cookware product. In water boundary conditions, there is a relatively small temperature variation (hot spot) on the heating surface, regardless of the amount of porous material. The temperature magnitude is relatively low compared to the air results due to the higher heat loss to the medium in contact with the heating source. The FEA runs with water boundary conditions assumed that the water was already boiling (110 C). The results indicated much lower hot spots than those indicated from the air runs. Thus concluding that the air boundary conditions are the worst case situation and that any cooking condition (soup, steak, etc.) would present fewer hot spots. With water boundary conditions, there is relatively small temperature variation (hot spot) on the heating surface, regardless of the amount of porous material.
With vessels with higher rims that would traditionally hold soups, stews, boiling water, pasta, etc. where the water boundary conditions apply, the maximum size porous metal foam and the minimum amount of face sheet would be incorporated. In these conditions the material inside the vessel is helping to heat the metals. Vessels intended for frying or saute with a 2xe2x80x3 rim height should incorporate a higher density metal foam with a minimum solid face sheet for optimum conductivity and minimum hot spots. All variations that incorporate metal foam decrease the weight of the cookware.